1. Field of the Invention
The present invention relates to a system and a method of delivering and metering a liquid, and more particularly to a system and a method of delivering and metering a liquid sterilant from a container into a vaporization system. The vapor or gas produced by the vaporization system is typically used for sterilization and/or decontamination purposes.
2. Background of the Invention
In order to sterilize certain devices or apparatus, particularly in the medical field, the device or apparatus can be placed in a chamber where liquid sterilant is vaporized. In order to ensure effective and efficient sterilization, the liquid sterilant must be metered in accurately and reproducibly measured amounts into the vaporization chamber.
One conventional method of metering liquid sterilant into a vaporization chamber involves extracting predetermined doses of liquid sterilant from a sealed cell. A cassette holds a group of these cells and in order to extract a dosage from each cell, a dispensing apparatus punctures each cell individually and pneumatic pressure drives the liquid sterilant out of the punctured cell.
This method presents several problems. First, using cassettes of cells offers little flexibility because the amount of liquid sterilant drawn into the chamber is limited to the individual cell volume, or multiples thereof. Secondly, in multi-phase or flow-through sterilization cycles where large volumes of liquid sterilant may be required, multiple cassettes are needed, making this method not only inflexible, but also uneconomical and impractical. Lastly, liquid sterilant (such as hydrogen peroxide) is susceptible to degrade into gases or vapors. When this degradation occurs, the gases or vapors may rupture the cassette cells unless the cells are vented. However, over time, venting reduces the concentration of the sterilant.
In another conventional method, the liquid sterilant is pumped from a reservoir into a vaporization chamber. The key to this method is the proper metering of the liquid sterilant in order to accomplish effective and efficient sterilization. Several control mechanisms exist to meter the proper amount of liquid sterilant, such as controlling the pump volume directly, controlling the revolution rate or dispensing time of a continuous flow, fixed output pump, and monitoring the weight loss of the reservoir as the liquid is pumped from the reservoir.
As with the conventional cassette method, these methods also suffer from difficulties associated with the degradation of the liquid sterilant over time. As discussed above, the liquid sterilant can degrade over time to form gases and vapors. Air bubbles created by the degraded gaseous sterilant will disrupt the effectiveness, efficiency, and accuracy of any of these control mechanisms. For example, air bubbles can cause a "vapor lock" in a stroke-type pump if it is allowed to remain idle for an extended period of time. Moreover, in a control mechanism which meters liquid sterilant by controlling the dispensing time period at a fixed pressure or vacuum, the liquid is pushed or sucked into the vaporizer, along with the air bubbles, in a non-uniform matter, causing significant decreases in efficiency and effectiveness. As a final example, the formation of gases and vapors will disrupt the effectiveness of a control mechanism which monitors weight loss from the liquid reservoir. When such a system remains idle for an extended period, the weight loss from the reservoir, as measured by the balance, will not account for the air bubbles formed in the dispensing lines, which are dispensed into the vaporizer at start-up.
In addition to the problems created when the liquid sterilant is allowed to degrade into gases and vapors over time, the conventional methods used to control the metering of the liquid sterilant face additional problems if they rely on high injection rates and high pressures. That is, in order to circumvent the problems of degradation described above, conventional control mechanisms apply high injection rates and high pressures in order to dispense the liquid sterilant as quickly as possible. However, these high injection rates and high pressures place an extra strain on the equipment and can often lead to system leaks. Moreover, due to the substances involved, compatibility problems may arise when attempting to reduce system leaks by constructing the equipment with certain types of material which can sustain such high pressures.
There is a need for a system and method of metering and delivering containers of liquid sterilant into a system which meters the liquid sterilant from a reservoir into a vaporization system. This process needs to be accomplished in accurately and reproducibly measured amounts. There is also a need for a metering system and method which can deliver a fixed and measured volume of the liquid sterilant into the vaporizer chamber at reduced flow rates to avoid system leaks and material compatibility problems. A flow sensor needs to be incorporated with such a system in order to achieve this objective.